pmem: Correctly account for aligned blocks

when finding an aligned chunk, the allocator cannot assume that
bit 0 will point to an aligned block. The only way to guarantee
a block will be aligned is to pass in the starting bit of a block
that is aligned. if t is the start bit which points to an aligned
block, and s is the spacing, bit t+n*s is guaranteed to be aligned
to spacing s for positive integer values of n.

CRs-Fixed: 288000
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>